Electro-optical scanners are widely used for reading bar codes, including one dimensional and two dimensional bar codes. A scanner typically includes a scanning module which: generates a scanning beam; repetitively directs and scans the beam across a target object, such as a bar code; receives reflected light from the target object; and digitizes and decodes the reflected light to decode the information encoded in the bar code. The scanning module is supported in a housing of the hand held portable scanner which also supports a power supply and other electronics of the scanner.
The scanning module scanning beam (typically a laser beam emitted by a laser diode) is directed at an oscillating scanning mirror. The oscillating scanning mirror, in turn, directs the beam outwardly through an exit window of the scanner. The exit window of the scanner functions to protect the internal components of the scanner supported within the scanner housing from the outside environment. The oscillation of the oscillating reflector causes the scanning beam to oscillate across a target object such as a bar code to be read. Essentially, the beam generates a beam spot that repetitively moves across or scans the bar code.
The light-colored or space elements of the bar code reflect the laser beam illumination and the dark or black bar elements of the bar code absorb the laser beam. Reflected light from the target bar code is received by a reflective surface such as a collection mirror and/or a lens and directed toward photodetector circuitry, such as a photodiode. The pattern of reflected light, as received by the photodiode of the scanning module, is a representation of the pattern of the bar code. That is, a sequence of time when the photodiode is receiving reflected light represents the laser beam spot moving across a space of the bar code, while a sequence of time when the photodiode is not receiving reflected light represents the laser moving across a dark bar. Since the scanning speed or velocity of the reciprocating movement of the laser is known, the elapsed time of the photodiode receiving reflected light can be converted into a width of a bar code element corresponding to a space, while the elapsed time of the photodiode not receiving reflected light can be converted into a width of a bar code element corresponding to a bar.
The photodiode is part of photodetector circuitry which converts the reflected light into an analog signal. The scanning module includes an A/D converter or digitizer to digitize the analog signal generated by the photodiode. The digitizer outputs a digital bar code pattern (DPB) signal representative of the bar code pattern. A decoder of the scanning module inputs the DPB signal and decodes the bar code. The decoded bar code typically includes payload information about the product that the bar code is affixed to. Upon successful decoding of the scanned bar code, the scanner may provide an audio and/or visual signal to an operator of the scanner to indicate a successful read and decode of the bar code. The scanner typically includes a display to display payload information to the operator and a memory to store information decoded from the bar code.
One type of electro-optical scanner, referred to as a retro-reflective scanner, employs retro-reflective light collection. In a retro-reflective scanner, the scanning module includes a mirror that both: 1) directs the scanning beam toward the target bar code or another mirror; and 2) receives reflected light from target bar code and directs it toward the photosensor circuitry. Another type of electro-optical scanner is referred to as a non retro-reflective scanner. In such a non retro-reflective scanner, the mirror that receives the reflected illumination and directs it toward the photodetector circuitry is physically separate from the mirror that directs the laser beam toward the target bar code.
A prior art portable electro-optical non retro-reflective scanner is shown at 200 in FIG. 1. The scanner 200 includes a housing 212 supporting a scanning module 220 which includes a laser diode light assembly 230 for emitting a scanning beam. The scanning beam (labeled SB1) is directed to a mirror 250 which repetitively oscillates about a vertical axis Y-Y through a scanning angle β. The redirected scanning beam (labeled SB2) exits the housing 212 through an exit window 218. Because of the oscillation of the mirror 250, the scanning beam SB2 is repetitively scanned in a horizontal direction forming a linear scan line SL.
Since the beam line SB2 is being scanned horizontally, the beam forms a horizontal scan line SL (FIG. 1) which extends across the bar code 100. Thus, a pie shaped scanning plane SP is formed emanating from the oscillating mirror 250. To scan and read a target bar code 100, the scan line SL is positioned to intersect the target bar code as shown in FIG. 1. Illumination or light from the scan line SL reflected from the target bar code 100 (labeled RSB in FIG. 1) passes through a light collection lens 260 and is focused on photodetector circuitry 240.
In non retro-reflective scanners, the light collection lens 260 (or system of lens) is typically employed to collect light reflected from the target bar code 100 and focus the reflected light rays RSB on the photodetector circuitry 240. As can be seen in FIG. 1, the light collection lens 260 is spaced from the scanning beam SB2, that is, the scanning beam SB2 does not pass through the light collection lens. One problem with the collection lens 260 of a non retro-reflective scanner stems from parallax effect. Because the collection lens 260 is spaced from the beam line SB2, an optical axis of the lens does not lie on the same axis as the scanning beam SB2 and, therefore, there is a parallax effect. An optical axis of a lens is defined as the straight line which passes through the center of curvature of the lens surface. A central ray though the scan plane SP is defined as the scanning beam line axis A-A.
For example, as is shown in FIG. 1, if the light collection lens 260 is disposed vertically above the scanning beam line axis A-A, because of the parallax effect, the light collection lens must be provided with an additional field of view (labeled FOV in FIG. 1) in a direction orthogonal to the beam axis A-A (that is, in the vertical direction) to be able to “see,” that is, receive and focus reflected light from the scan line SL as it traverses or intersects the bar code 100. This is because the scan line SL intersects the bar code 100 at a vertical level that is below the level of the lens 260. Thus, the light collection lens 260 must look downwardly to “see” reflected light from the scan line SL impinging upon the target bar code 100. Stated another way, the light collection lens 260 must have a vertical field of view to “see” reflected light from the scan SL when scanning bar codes which are close in proximity to a forward nose N of the scanner and bar codes which are more distant. Unfortunately, an unfortunate result of increasing the vertical field of vision of the light collection lens 260 more ambient light (that is, light not reflected from the scan line SL) is focused on the photodetector circuitry decreasing the signal-to-noise ratio and generally making it more difficult to successfully decode the target bar code 100.
What is desired is a light collection lens system that would substantially eliminate the parallax effect in non retro-reflective scanners. What is also needed is a light collection lens system that would eliminate or reduce the need for additional FOV in a direction orthogonal to the scanning beam axis. What is also needed is a light collection lens system that reduces the amount of ambient light focused on the photodetector circuitry.